In CDMA-based systems, narrowband signals (as represented by a sequence of symbols) are translated into signals with wider bandwidth (as represented by a sequence of chips) via a technique known as spreading. In simplified terms, a symbol to be transmitted is converted into a longer sequence of chips. This sequence is constructed by multiplying each complex symbol with the time varying complex spreading code sequence, which is different for each channel.
The so-called PRACH (Physical Random Access Channel) is a UMTS (Universal Mobile Telecommunication System) uplink common physical channel, i.e. it is shared between all user equipment (UE) within a cell. Each user equipment in the cell utilizes the PRACH to send signaling information such as a call origination requests to the UTRAN (Universal Terrestrial Radio Access Network) and, if necessary, a small amount of user data, such as short messages, alphanumerical texts, and so on. Every UE that wants to use the PRACH randomly selects an access slot number and a 16 chip long signature code, see FIG. 1. Each access slot is 5120 chip long, and the timing of the access slots is defined relative to the AICH (Acquisition Information Channel) Downlink Channel. The signature code is any one of the 16 Walsh codes of length 16.
The cell can be set up with a subset of valid access slots and signature codes. In that case, the UE has to select from the valid subset.
In general, during the selected access slot, the UE sends a combined code obtained by multiplying the 4096 chip of the PRACH scrambling code with the 16 chip signature repeated 256 times, as illustrated in FIG. 2.
At a receiving unit, the combined code needs to be subjected to preamble detection, other chip rate operations such as cell searching and despreading.
Depending on the position of the UE in the cell the delay of the UE, compared to the system time, will vary. Since the position initially is unknown, the uncertainty in delay is equal to the entire cell radius. The preamble detection part of RACH is basically a correlation based synchronization procedure that both detects the presence of a UE and estimates the delay compared to the access slot. Due to the uncertainty in delay the 4096 chip that constitutes the preamble code according to system time is extended withW=2*R/78+β[chip]  (1)corresponding to a search window W equal to the cell radius R, the term β represents an uncertainty in the window.
These Wextended=4096+W [chip] are then correlated with the combined codes for all possible delays and all valid signature codes to form one Power Delay Profile (PDP) per signature code. The PDP peaks of each signature is then validated against a threshold, and if a valid peak is found the corresponding Round Trip Delay (RTD) of the user is calculated from the delay corresponding to this peak.
In case of a varying radio channel, summing coherently over all 4096 chip is not optimal. The PDP formatting part of the algorithm is then typically performed separately for small sections of the 4096 chip sequence. The PDP:s from all sections are then summed non-coherently before peak validation.
The length of such a section is given byN*16  (2)where N is an integer that divides 256.
In any known commercial hardware for WCDMA base band processing, the huge complexity of the preamble detection forces the use of dedicated hardware accelerators HWA or so-called co-processors for the above described correlation operations.
At present, there are different types of HWA dedicated for preamble detection. Some are highly specialized HWA that cannot perform other chip rate tasks such as synchronization of other physical channels, path searching, or symbol despreading in a Rake receiver. Some are general purpose HWA that perform correlation or despreading with general spreading codes, and can be used for any CDMA chip rate task.
There is also a division between external HWA, e.g. ASICs connected to the DSP, loosely coupled HWA and tightly coupled HWA. For loosely coupled HWA, the antenna data, that constitutes the input to the HWA, does not enter the DSP cores. Tightly coupled HWA are computational units in the DSP:s that support chip rate task instructions, and the antenna data enters these units as inputs to special functions called from the DSP.
Regardless of any of the above implementations, there is a need for improved methods and arrangements for receiving and decoding the PRACH combined code with reduced complexity and increased speed.